Since the demonstration of low working voltages [compare Tang et al.: Appl. Phys. Lett. 51 (12), 913 (1987)], organic light-emitting diodes have become promising candidates for the realisation of large-surface displays and other applications such as illuminating elements. They comprise an arrangement of thin layers of organic materials. Either, the layers are preferably vapour-deposited in a vacuum in the form of molecules. Low-molecular layers are formed in this way. An organic light-emitting diode on the basis of low-molecular layers formed in this way from separable molecules by means of vacuum evaporation is designated as an OLED in its abbreviated form. Reference is also made to the “small molecule” technology field in this connection.
Alternatively, the layers consisting of materials are formed from polymer materials which are spin-coated from a solution, printed or applied in any other suitable form, so that polymer (organic) layers are established. An organic light-emitting diode on the basis of polymer layers formed in this way is also designated as PLED in the abbreviated form.
With the injection of charge carriers, namely of electrons and holes, from the electrode contacts into the organic layers arranged in between when placing an external voltage to the electrode contacts, of the following formation of exitones (electron-hole couples) in an active light-emitting zone (emission zone) within the organic layers and the radiating recombination of the exitones, light is generated and emitted from the device.
Organic light-emitting diodes with a PLED-configuration are normally based on the following layer structure: (1) carrier substrate (transparent, made from glass for example); (2) anode contact (transparent, usually made from indium tin oxide (ITO)): (3) hole transport or hole injection layer (for example from PEDOT:PSS or PANI—polyaniline with admixtures such as PSS; PEDOT=polyethylene dioxythiophene, PSS=polystyrene sulfonate); (4) polymer layer as a light-emitting zone from a polymer material (for example MEH-PPV, polyfluorenes, other PPVs, polyspiros, polythiophenes or polyparaphenylenes) and (5) cathode contact (for example from a metal with low work function such as barium, calcium).
The polymer layers, namely the hole transport or hole injection layer and the light-emitting zone are manufactured from a liquid solution, for example in water or in solvents. The electrode contacts (anode and cathode contact) are produced typically by means of vacuum processes.
The advantages of this structure of an organic light-emitting diode for applications, displays for example, are the diversity of the processes for forming the polymer layers. Included here are such processes which allow a plain lateral structuring of the PLED, namely the inkjet printing technique. With this method different types of polymer materials are printed onto previously treated locations, through which adjacent areas of different emission colour can originate. Other structuring methods include the screen printing technique.
The disadvantage of the known PLED structures is, among other things, the fact that not more than two different polymer layers can be deposited in a suitable manner because the solvents of the polymer materials must be selected in such a way that they do not influence each other, meaning, that they do not attack the material of the substructure. This means that the deposited polymer material must be simultaneously good for the electron transport and must be suitable for the electron injection from the cathode contact, a requirement which is a major restriction for the material selection and the structure optimisation. Recent examinations have shown that three-layer structures are also realisable.
In addition, the sequence of the structure for a given material system can be changed with a great amount of difficulty only. As described above, therefore, the anode contact is the starting point.
This is particularly disadvantageous for the integration of the PLED structure on active matrix display substrates with n-channel transistors as a contact element. The use of transparent top contacts is also difficult because these (i) have an unfavourable work function for the electron injection (work function is too great) and (ii) are usually manufactured by means of a sputtering process. However, this process destroys organic materials. As the upper layer in a PLED is a light-emitting layer, the efficiency of the light generation of the organic light-emitting diode is reduced as a result. In order to improve the stability against sputtering damage the application of a low-molecular organic layer, vapour-deposited in a vacuum, consisting of small molecules was envisaged. In this ease also, however, the electron injection from the cathode contact is a problem.
A further disadvantage of the conventional PLED structure is the fact that an efficient electron injection can be obtained with only very unstable contact materials such as barium or calcium. These materials, however, are attacked by oxygen and water. Moreover, it is very difficult to use one and the same electrode (cathode) for all emitter materials of the three basic emission colours red, green and blue because this involves considerable performance losses with one of the three colours. An optimised cathode contact for the blue-emitting polymer material has disadvantages for the red-emitting polymer material, and vice versa.
Organic light-emitting diodes with an OLED-configuration which are allocated to the field of the “small molecule” technology have, as organic structures, layers from molecules that are vapour-deposited in a vacuum where said molecules are of one or several organic materials. If the molecules of the organic material are small enough, they can usually be deposited without decomposition by means of a thermal process. For this purpose, the molecules are evaporated in a vacuum.
A typical structure of an organic light-emitting diode with OLED configuration (maximum configuration) is as follows: (1) carrier substrate (glass, for example); (2) anode contact (hole-injecting, preferably transparent, made from indium tin oxide (ITO), for example); (3) hole-injecting layer (for example from CuPc (copper-phthalocyanine) or starburst derivatives); (4) hole transport layer (for example from TPD (triphenyldiamine and derivatives); (5) hole-side blocking layer (in order to prevent exitone diffusion from the light-emitting area and to prevent charge carrier leakage from the light-emitting area, for example from Alpha-NPB); (6) light-emitting area (for example CBP with emitter admixture (for example iridium-tris-phenylpyridine Ir(ppy)3)); (7) electron-side blocking layer (in order to prevent exitone diffusion from the light-emitting area and to prevent charge carrier leakage from the emission area, for example from BCP (bathocuproine)); (8) electron transport layer (for example from Alq3 (aluminium-tris-quinolate)); (9) electron injection layer (for example from inorganic lithium fluoride (Lin); and (10) cathode contact (electron injecting, usually made from a metal with low work function, aluminium for example). The design as described comprises a maximum number of possible layers. In other designs, layers can be dispensed with. One layer can also take over several functions. For example, the hole injection layer and the hole transport layer or the hole transport layer and the hole-side blocking layer or the hole injection layer, the hole transport layer and the hole-side blocking layer can be put together. There is furthermore the option of mixing the materials of the electron injection layer into the electron transport layer.
With the OLED configuration there is also the option of envisaging doped transport layers with electric doping for improving the electric conductivity. Their general and typical structure is as follows: (1) carrier substrate (glass, for example); (2) anode contact (hole-injecting, preferably transparent, made from ITO for example, but also from Ag, Au and as another reflecting contact); (3) p-doped holes injecting and transporting layer (the dopant is then an acceptor material which is capable of taking over electrons from a matrix material, for example from m-MTDATA doped with F4-TCNQ, for further acceptor dopants refer to U.S. Pat. No. 6,908,783 B1); (4) hole-side blocking layer (from a material whose tape layers match the tape layers of the layers surrounding it, so that exciplex formation between holes on the p-doped hole injecting and transporting layer and electrons in a light-emitting area is prevented; alpha-NPB, for example); (5) light-emitting area (for example from TCTA with emitter admixture, for example iridium-tris-phenylpyridine Ir(ppy)3)); (6) electron-side block layer (typically thinner than the following named layer; from a material whose tape layers match the tape layers of the layers surrounding it, so that exciplex formation between holes on the p-doped holes injecting and transport layer and electrons in a light-emitting is prevented; for example from BCP); (7) n-doped electron injecting and transporting layer (the dopant is then a donor which is capable of transmitting additional electrons onto a matrix material; for example from BPhen—bathophenanthroline doped with caesium as inorganic dopants or W2(Xpp)4 (tetrakis(1,2,3,3a,4,5,6,6a,7,8-decahydro-1,9,9b-triazaphenalenyl)ditungsten(II); for further dopants refer to US 2005/0040390 A1, US 2005/0061232 A1, WO 2005/036667 A 1, WO 2005/086251 A3); and (8) cathode contact (electron injecting, usually made from a metal with low work function, for example Al, but also from Ag, Au).
At the beginning of the evaporating process, the dopants in the doped layers do not have to be in their final form as long as an alternatively applied precursor material forms the dopant during the evaporating process, which can also be modified, for example with the use of electron beams. The manufacture of the mixed layers is typically effected by means of mixed (co-) evaporation.
The advantages of such an OLED configuration are its higher light generation efficiency as well as the life service and the variance of the structure. The longer life service of the devices with OLED-configuration compared to the devices with PLED-configuration is explainable by the higher degree of purity of the applied organic materials as obtained with vacuum cleaning methods. Advantages are furthermore the separated optimising capability of the properties of the individual layers, the adjustably large clearance of the light-emitting area to the electrode contacts. With doped devices of the pin-OLED-type, there are also a low operating voltage and a variance of the electrode materials. As presented, for example, in the document US 2004/0251816 A1 and in Zhou et al. (Appl. Phys. Lett. 81, 922 (2002)), this structure can also, and in addition, be easily inverted and can be made top-emitting or fully transparent (compare US 2006/0033115 A1).
The disadvantage of such devices is that a lateral structuring of the OLED-structure for the configuration of differently coloured pixels in a display is normally performed with shadow masks. This process has limitations with regard to the smallest obtainable pixel sizes which are less than approximately 50 μm. Shadow masking in a manufacturing process involves a relatively considerable work effort. The inkjet process as used for depositing polymer materials is either not useable or only useable with limitations for small molecules during the formation of low molecular layers due to the non-solubility of the organic materials of the small molecules. LITI (“Laser Induced Thermal Imaging”) is an alternative process which, on its part, has limitations particularly with the selection of the process-compatible materials.
Furthermore, organic light-emitting devices with a hybrid structure are known which are also designated as hybrid organic devices for this reason. In the document US 2003/020073 A1 the use of vapour-deposited low-molecular blocking layers and electron transport layers on a polymer hole transport layer is described. However, with this arrangement the injection of charge carriers, namely electrons from the cathode contact into the low-molecular electron transport layer, is problematic. The operating voltage of the device is increased as a result.
The document WO 2005/086251 focuses on the use of a metal complex as an n-dopant for an organic semi-conducting matrix material, an organic semiconductor material and an electronic device as well as a dopant and a ligand.
In the document EP 1 511 094 A2, a light-emitting device is disclosed wherein organic molecule layers and polymer layers are envisaged.
The properties of the different materials involved can be described in the PLED-configuration and in the OLED-configuration by the energy levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). Hole transport layers, including related blocking materials, usually have HOMOs in the range of 4.5 eV to 5.5 eV under vacuum level, LUMOs in the range of 1.5 eV to 3 eV. With materials for the light-emitting range, the HOMOs lie in the range of 5 eV to 6.5 eV and the LUMOs in the range of 2 to 3 eV. With materials for electron transport layers, including suitable blocking materials, the HOMO lies in the range of 5.5 eV to 6.8 eV and the LUMO in the range of 2.3 eV to 3.3 eV. The work functions for the extraction of electric charge carriers with the materials for the anode contact lie in the range of 4 eV to 5 eV and for the cathode contact in the range of 3 eV to 4.5 eV.